Front shifting issue

JonEdwards
JonEdwards Posts: 452
edited October 2009 in Workshop
Maybe 1 front downshift in 50 (or more) I get an absolutely horrendous change. As far as I can tell the chain derails fine until the last tooth which it then sticks to and jams before tearing off with an awful crunch and removing a lot of paint from my frame at the same time.

It seems to be completely unpredictable - sometimes happens under load, sometimes when I'm just turning the cranks to make the change. I've been over the chain and rings with a fine toothcomb and can't spot any bent or twisted bits, or even any graunches from the change which would point towards the culprit. I can't replicate it on the work stand either. Both the chain and the ring are in good nick (chain only has a couple of 100 miles on it) and it also happened with the previous chain (diffferent make). It also happens so rarely/randomly that I'm never actually expecting it and it always catches me by surprise.

It's obviously not doing the frame, the rear mech (I have some chipped paint from the mech cage hitting the stay), or the chain and ring any good, and it's only a matter of time before the whole damn thing locks up and I end up throwing myself over the bars.

Any ideas? If it was an MTB, the usual diagnosis would be an obviously worn chain or trashed ring, but I really can't see any sign of any damage at all...

Chorus rings, KMC X10sl chain, Record mechs and shifters.

Cheers,

Jon

Comments

  • Wooliferkins
    Wooliferkins Posts: 2,060
    Something awry with the front mech maybe? I'm just throwing ideas about. Maybe if the front mech is a smidge too close to the chain ring it occasionally catches between teeth and cage? Odd one good luck.
    Neil
    Help I'm Being Oppressed
  • John.T
    John.T Posts: 3,698
    That is chain suck. Not a lot you can do about it if your chain set is prone to it. Try to keep it all clean and well lubricated and change gear with care.
    It is more common on MTBs due to muckier operating conditions.
  • That is chain suck

    Yer not wrong there.

    Thing is - chain suck on MTBs is usually pretty consistent. Once you get it, you carry on getting it. You know every change is going to be bad, so you can compensate for it. This shifts perfectly most of the time, which is what's puzzling me - I keep expecting to find one bent tooth or something.

    Oh, and it's all clean and well lubed!

    Might have to try sticking a new big ring on... :?
  • peanut
    peanut Posts: 1,373
    if its that intermittant it could be one tight or stiff chain link . I'd remove the chain clean it lube it and check every single link which should be nice and floppy loose . You may need to adjust a pin somewhere. Do you use a tool-less link ? is it the correct size ?

    other than that I'd check that the front mech front cage plate is exactly in line with the big ring and 2-3mm above the teeth.

    oh and worth checking your chain alignment. BB axle might be too short or long for the shell ?